Accelerated mineralization of textile wastewater under 222 nm irradiation from Kr/Cl2 excilamp: an environmentally friendly and energy efficient approach

The textile dyeing and manufacturing industry is the major producer of significant amounts of wastewater that contain persistent substances such as azo dyes that require adequate remediation measures. Far ultraviolet at 222 nm light may provide an advantage for contaminants degradation as compared to conventional UV sources (254 nm). In this paper, the degradation of reactive black 5 (RB5) in artificial wastewater has been performed using a 222 nm Kr/Cl2 excimer source under direct photolysis and an advanced oxidation process using TiO2/H2O2. The solution pH, catalyst concentration, 222 nm intensity, initial concentration of dye, and addition of H2O2 influence the degradation rate constant. The molar absorption coefficient, quantum yield of RB5 at 222 nm and the electrical energy per order (EEO) from different treatment methods have been reported. RB5 shows 1.26 times higher molar absorption at 222 nm than at 254 nm. The EEO for excimer-222/H2O2 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document}∼ 13 kWh/m3) is five times lower than that of the excimer-222/TiO2 process, which makes the process energy efficient. The degradation of wastewater has been carried out at three distinct pH values (2, 6, and 10), and the pH level of 10 exhibited the highest degree of degradation. The degradation rate in the alkaline medium is 8.27 and 2.05 times higher than in the acidic or ambient medium. Since textile effluent is highly alkaline, this result is significant, as no neutralization of the wastewater is required, and direct treatment is possible. A possible degradation pathway has been established based on Fourier transform infrared spectroscopy (FTIR) and high resolution mass spectroscopy (HRMS) analysis. The phytotoxicity of the treated wastewater has also been evaluated for its suitability for reuse in agriculture. The study reveals that the excimer-222/H2O2 treated wastewater significantly enhanced the germination percentage of Raphanus sativus seed (97%) compared to dye wastewater-grown seeds (75%). This work offers crucial information for future studies on the direct and indirect photolysis of azo dyes, as well as insight into the process of RB5 degradation under Kr/Cl2 excimer radiation.

2. A Kr/Cl 2 excilamp has been developed and optimized which emits 222 nm UV-C light and will be safer for human exposure.A narrow and intense spectral band has been obtained that can be utilized for the photolysis of H 2 O 2 for the enhanced production of hydroxyl radicals.3.By adding H 2 O 2 as a radical promotor (less than 10 ppm) in excimer-222/AOP, accelerated degradation of organic dye is reported as compared to the photocatalyst loading.This process has eliminated the recovery step of catalysts from the treated wastewater.4. Instead of utilizing DI water as a solvent, RB5 dye has been dissolved in sea water matrix for the practical applicability of the proposed method.A degradation mechanism under excimer-222/H 2 O 2 has been discussed to adequately understand the degradation reaction kinetics.5.One of the primary distinctive features of this study is to utilize the post-treated wastewater for phytotoxicity analysis on seed germination to assess its prospective use in agriculture.

Materials and far UV-C 222 nm source
The information about the different materials used in this study is included in Sect.1.1 of the supplementary information (SI).The information on the development process of the Kr/Cl 2 excilamp is explained in our previous work 27 and also provided in Sect.1.2 of SI.
Figure 1a shows the 2-D schematic and original view of the developed Kr/Cl 2 (222 nm) excilamp.When subjected to high voltage, the excimer source triggers gas excitation, producing KrCl* excimer molecules.In an instant of nanoseconds after excitation, the excimer molecules undergo decomposition and subsequently emit ultraviolet (UV) photons with a specific wavelength of 222 nm, accompanied by weaker transitions occurring at various wavelengths.The developed excilamp had a length of 15 cm and an outside diameter of 1.6 cm.This measurement provides a surface area of approximately 75 cm 2 , adequate for emitting UV radiation.The optical parameters of the exciplex source were analysed using an Andor PMT-based monochromator (SR-500i-B1) equipped with an optical fibre (200 µm, ocean optics).The optical spectra of the far UV-C exciplex source from 200 to 300 nm were recorded using a grating with 2400 grooves/mm.For the measurement of the incident irradiation E 0 (in mW/cm 2 ) from the excimer source in air, a Hamamatsu make UV power meter (C9536) has been used with a sensor head (H9535-222) pre-calibrated at the 222 nm wavelength.The average UV fluence rate ( E avg ) delivered to the wastewater samples has been calculated by including all the factors that influence the intensity i.e., water factor (WF), petri factor (PF), divergence factor (DF), and reflection factor (RF) as shown in the equation, All these parameters are calculated using the standard method given by Bolton and Linden 28 , and reported as WF is 0.985, PF is 0.973, RF is 0.90.A complete experimental setup, for the optical and electrical characterization of the developed Kr/Cl 2 excimer source is shown in Fig. 1b.The influence of phase and crystalline size on the photocatalytic activity of a catalyst is significant 29,30 .The TiO 2 nanoparticles were synthesised using the sol-gel technique, and a detailed procedure for their preparation is explained in our previous studies 31,32 .To confirm the formation of nanoparticles, the TiO 2 suspension was subjected to further annealing at a temperature of 450 °C for 3 h.The resulting powder was then examined using X-ray diffraction (XRD) and X-ray photon spectroscopy (XPS).
The Tauc equation and Kubelka-Munk technique were employed to determine the optical bandgap ( E g ) energy of the synthesised nanoparticles 33 .XRD (D-8 Advance, Bruker) equipped with Cu-Kα radiation (λ = 1.54 Å) was performed to confirm the phase of the TiO 2 nanoparticle, data were taken between 2 theta values of 20° and 80° with a step size of 0.02°.The field emission scanning electron micrograph (FE-SEM, Apreo 2, Thermo Fisher Scientific, USA) and high-resolution tunnelling electron microscope (HRTEM, Talos F200X G2, Thermo Fisher Scientific, USA) confirms the crystalline size of the synthesized nanoparticles.The FE-SEM characterization has been performed with 10 kV high voltage in secondary electrode (SE) mode.X-ray photoelectron spectroscopy (XPS: Monochromatic Al-Kα source, Thermo Fisher Scientific, USA) studies were performed to observe the surface property and chemical states of the synthesized material.For the XPS characterization, monochromatic Al-Kα source (hv = 1486.6eV) with a hemispherical analyser and a 128 channel plate detector have been used.

Procedure and analysis of pollutant degradation
The DBD Kr/Cl 2 excilamp was employed to study the degradation and mineralization of RB5 dye in artificial wastewater under different conditions.To validate the developed reactor for industrial applications, artificial wastewater was used for the experimentation.For the same, diluted sea water (pH = 10, conductivity ∼ 7370 µS/ cm, and TDS ∼ 3850 ppm) is treated with excimer 222 and LPUV with TiO 2 and H 2 O 2 loading.Total dissolved solids (TDS) refers to the quantitative measurement of all inorganic and organic components that are present in a liquid in molecular, ionized, or micro-granular suspended form.Generally, the electrical conductivity of textile wastewater was found to be in the range of 4400-8700 µS/cm.Diluted sea water was considered as a surrogate of textile wastewater because of the presence of various salts.Furthermore, textile industries uses various salts for the wet dying process to enhance the penetration of dye molecules into the fabric as well as for the enhanced dying efficiency 34 .In the present study, diluted seawater along with RB5 dye is considered as a textile wastewater.
A solution containing different concentrations of RB5 dye (10 mg/L, 50 mg/L, and 90 mg/L) in artificial wastewater was chosen as a model pollutant.RB5 is chosen for this study because of its complex chemical structure as well as its molar absorption coefficients and quantum yield is unknown for 222 nm.An acrylic chamber of one cubic feet was used to integrate the Kr/Cl 2 excilamp for dye degradation experimentation.100 ml of artificial wastewater solution was treated for each test and 2 ml samples from suspension were withdrawn at regular time intervals and were immediately centrifuged at 3000 rpm for 15 min to completely remove catalyst particles if they were loaded.A calibration curve for known RB5 concentrations is plotted using the standard solution (see Figure S1) to measure the amount of RB5 after different processes.
To optimise the loading of the catalyst, varying concentrations of TiO 2 catalyst (0.5 g/L, 0.75 g/L, 1 g/L, 1.25 g/L, and 1.5 g/L) were introduced.The resulting suspension was then magnetically stirred for 30 min in a dark environment.This was done to ensure the thorough equilibration of the adsorption and desorption of RB5 on the surface of TiO 2 .In most cases, studies were conducted at ambient pH of the solution during the photocatalytic reaction.To alter the initial pH for acidic or alkaline studies, the appropriate amount of H 2 SO 4 or NaOH was added accordingly.The experiments were conducted under standard temperature and pressure conditions.The transfer of radiative heat from the excilamp to the treated suspension can be computed using fluid modelling studies 35 .Each experiment was replicated multiple times to ensure preciseness, and the median values were plotted, wherever possible.Kruskal-Wallis test was performed to compare quantitative variables which were not normally distributed.
The concentration of RB5 dye at its maximum absorbance ( max = 598 nm) was measured using a UV-Vis spectrophotometer (Cary 4000, Varian).High-resolution mass spectrometry (HR-MS) and Fourier transform infrared spectroscopy (FTIR) were employed to analyse the potential degradation mechanism.The COD test has been used to investigate dye mineralization by measuring the oxygen demand for the breakdown of organic pollutants.COD is used in wastewater treatment plants to quantify the amount of oxygen needed to decompose organic matter (pollutants).
The molar absorption coefficient (ε) and quantum yield (Ø λ ) for RB5 dye are determined using the following relationships 36 , where A is the absorbance of the sample at a particular path length, C is the concentration of target compound (M), l is the path length (cm), k(λ) is the pseudo first order rate coefficient (s −1 ), E avg (λ) is the average incident photon irradiance.The pseudo-first-order kinetic model for RB5 degradation is shown in Eq. (7).The dye degradation efficiency (%), EEO, and COD reduction are measured using the following expressions, where C 0 and C t are the initial and final concentrations of the RB5 dye (in mg/L), P is the consumed power of Kr/ Cl 2 excilamp (in kW), V is the treated suspension volume (L), t is the treatment/exposure time (in hour), COD 0 and COD t are the initial and final value of chemical oxygen demand (in mg/L), k is the first order rate coefficient.

Evaluation of the reusability and toxicity of the dye wastewater
The reusability of treated wastewater was evaluated by germination of Raphanus sativus seeds to test the toxicity of untreated and treated artificial organic wastewater.The process of seed germination was investigated utilising various water samples, including deionized water, artificial wastewater, LPUV DP, excimer-222 DP, excimer-222/ TiO 2 (1 g/L), and excimer-222/H 2 O 2 (10 ppm) treated artificial wastewater.Prior to employing the treated solutions for germination, their pH was neutralized.The Raphanus sativus seeds were planted directly in the soil, and untreated and treated wastewater were used to irrigate the soil.The purpose was to observe the impact of various water samples on the growth of the shoot and root of the seeds.This study was conducted over a period of 3 d to investigate the effects of treated and untreated organic wastewater on the germination rate, root length, and shoot length of Raphanus sativus seeds.Germination percentage has been calculated by using the following relationship,

Excilamp characterization
Figure 2a shows the typical voltage and current waveforms of the DBD Kr/Cl 2 excilamp at an applied parameters of 9 kV/25 kHz.The DBD Kr/Cl 2 excilamp discharge power is calculated by multiplying the measured voltage and current values, followed by their integration over a given time period 37 .At optimized parameters, the discharge power of the DBD Kr/Cl 2 excilamp is found to be 15.23 W with 1050 µW/cm 2 of UV output.A further increase in input power linearly increases the UV output of the DBD Kr/Cl 2 excilamp.Figure 2b shows the optical emission spectra (OES) of the DBD Kr/Cl 2 excilamp.A prominent spectral band at 222 nm has been observed, which corresponds to the B-X transition of KrCl* excimer.Additionally, there are weaker transition bands observed at 235 nm (B-A transition of KrCl* excimer), 240 nm (C-A transition of KrCl* excimer), and 258 nm ( Cl * 2 transition).In addition to the 222 nm spectral band, these weak bands contribute to the breakdown of contaminants by promoting the production of active radicals.
The UV power density (absolute intensity at 222 nm) of the developed DBD Kr/Cl 2 excilamp as a function of discharge power (W) is shown in Fig. 2c.It is observed that the intensity exhibits a linear relationship with the increase in discharge power (R 2 > 0.99), and it takes around 30 s to stabilise the UV intensity.Until specifically mentioned, the studies addressing the degradation of RB5 dye were conducted with an absolute UV intensity of 1050 mW/cm 2 , which was optimised based on gas pressure, applied voltage, and excilamp heating.The temporal variation of temperature of the treated dye suspension in relation to the far UV-C fluence rate is depicted in Fig. 2d.Even after 3600 mJ/cm 2 (or 60 min) of excimer 222 nm exposure, the temperature of the dye solution remains well below 315 K, indicating the non-thermal nature of excilamp and the treated suspension.

Characterization of the material
The XRD pattern of TiO 2 nanoparticles is shown in Fig. 3a.The most intense characteristic peaks in the XRD pattern at 25.21° which corresponds to the (101) crystal plane of TiO 2 .The minor peaks appeared at 36.97°, 37.78°, 38.52°, 48.01°, 53.94°, 55.01°, and 62.64°, which correspond to (103), (004), (112), ( 200), (105), (211), and (213) crystal planes of TiO 2 , respectively 38 .The XRD spectra of synthesized material reveal the pure anatase phase of TiO 2 nanoparticles.Further, the diffuse reflectance spectra were recorded to investigate the optical bandgap of the TiO 2 .The Kubelka-Munk function is represent as , where F(R) and R represent as comparable to the absorption coefficient ( α) and mode of the reflectance and (F(R)hν) 1 2 vs hν plotted and shown in Fig. 3b.The power ½ reperesents the indirect band gap nature for the synthesised materials.To get the band gap, the linear extraplottation of the (F(R)hν) 1 2 = 0 drawn on the x-axis.The obtained band gap for TiO 2 is 3.19 eV which corresponds to the anatase phage 39 .According to the literature, this phase of TiO 2 nanoparticles is the most suitable for photocatalytic applications in degrading dyes 40 .To evaluate the size and morphological characteristics of the synthesized nanoparticles, the TiO 2 nanopowder was subjected to FE-SEM analysis.The resulting image is depicted in Fig. 4a.The particle size of TiO 2 was analysed using the image view software, with measurements conducted for more than 60 particles.Figure 4b display the historiographical depiction of this measurement.The Gaussian distribution was employed for size analysis.The nanoparticles have an average size of ∼ 24 ± 8 nm, as seen in Fig. 4b, making them appropriate for  www.nature.com/scientificreports/photocatalytic applications 41 .These distribution and XRD measurements confirmed that the synthesised TiO 2 material are in the nanometer regime.The size of synthesized nanoparticles were also confirmed by the HRTEM analysis, and the average size was found between 25 and 35 nm (as can be seen from Figure S2).
The surface property and different chemical states of TiO 2 nanoparticles have been evaluated using XPS. Figure 4c and d shows the XPS spectrum recorded for the synthesised TiO 2 .Two spectrums are observed: one for O 1s and the other for Ti 2p.The XPS spectrum of Ti 2p is depicted in Fig. 4c, displaying two distinct doublet peaks: Ti 2p3/2 and Ti 2p1/2, with binding energies of 458.08 eV and 463.99 eV, respectively.The presence of this doublet can be attributed to the spin-orbit coupling interaction 42 .The primary peak observed at 529.68 eV in Fig. 4d is attributed to the presence of TiO 2 .A broad peak at 531.18 eV may indicate that the adsorbed hydroxyl and oxygen ions are weakly bonded.In addition to Ti and O, no additional peaks are observed, indicating the high degree of phase purity of the synthesised TiO 2 nanoparticles.

Characteristics of RB5 dye under different wavelengths
Figure 5 shows the molar absorption spectra of the 50 mg/L RB5 dye suspension.It can be clearly seen from Fig. 5 that the molar absorption coefficient of RB5 dye at 222 nm (21,382 M −1 cm −1 ) is greater than 254 nm (16,870 M −1 cm −1 ), which means the RB5 dye will degrade 1.26 fold faster under 222 nm than 254 nm in case of direct photolysis.Also, the quantum yield at 222 nm is found be 0.0502, whereas the quantum yield at 254 nm is found to be 0.0018, which holds good promise for faster degradation under excimer 222/AOP.The molar absorption coefficient and quantum yield of RB5 at 222 nm and 254 nm wavelengths can be seen from Table S2.
A blank adsorption experiment was conducted in the dark, wherein a concentration of either 1 g/L of TiO 2 or 10 ppm H 2 O 2 was added to a dye suspension with a concentration of 50 mg/L.The results indicated minimal degradation (less than 2%) in the initial concentration of RB5.A comparable experiment was conducted under visible light conditions, revealing a decrease of 4% in the starting dye concentration after 60 min.This suggests that the utilized dye exhibits photostability within the visible spectrum, aligning with the earlier findings 43 .mJ, with an increase in the catalyst loading from 0.5 to 1 g/L.By doubling the catalyst dose (from 0.5 to 1 g/L), the degradation rate constant becomes threefold.As the TiO 2 loading increases, the absorption of UV-C photons also increases, leading to a higher production of •OH and an increase in the absorbance of dye molecules 43 .From this study, the optimal concentration of TiO 2 catalysts for the degradation of 50 mg/L dye has been determined to be 1 g/L under DBD Kr/Cl 2 excilamp with a degradation rate constant of 1.41 × 10 -3 cm 2 /mJ.The reusability and stability of the synthesized catalyst have also been tested for up to six cycles (see Figure S3).The results reveal that the synthesized catalyst can be used for a number of cycles, demonstrating its stability and reusability in the practical wastewater applications.
Figure 7 shows the photocatalytic mechanism action of TiO 2 in organic pollutants treatment under the irradiation of Kr/Cl 2 excilamp.The electrons and holes generated by photochemical processes undergo reactions with the oxygen and water molecules in their vicinity, forming superoxide radicals ( O •− 2 ) and •OH.After a series of reactions, the O •− 2 will subsequently generate •OH.The organic substance will decompose into carbon dioxide, water, and various other by-products due to the action of •OH.A detailed reaction mechanism of TiO 2 has been provided in the SI in Sect.1.3.

Effect of addition of H 2 O 2
Figure 6b shows the UV-Vis profile of RB5 dye solution before and after treatment of excimer 222/H 2 O 2 .A weak band at 312 nm (in the UV-B region) has also been observed after decolourization, which is probably due to the presence of aromatic by-products in the reaction medium 43 .For 2 ppm concentration of H 2 O 2 , the degradation rate constant obtained is 1.48 × 10 -2 cm 2 /mJ, whereas with further increase in the H 2 O 2 concentration i.e. up to 8 ppm, the degradation rate does not show significant variations.Finally, the concentration of H 2 O 2 increases to 10 ppm which is close to the limit of municipal UV/H 2 O 2 systems 44,45 , a significant improve in the rate constant (four times faster than 2 ppm) was observed.Figure 6d shows the effect of different loading concentrations of the H 2 O 2 over the degradation profiles of RB5.The degradation rate constant for RB5 in the case of excimer-222/H 2 O 2 is 4.16 × 10 -2 cm 2 /mJ, which is 29.5 times faster than the excimer-222/TiO 2 .From Fig. 6e, if we compare the DP of RB5 under LPUV and 222 nm, then 222 nm is more effective than LPUV due to its ability to produce more •OH in water from the nitrates 19 .The degradation rate constant obtained in DP of RB5 under excimer 222 nm is 4.76 × 10 -4 cm 2 /mJ.Upon comparison of the excimer-222/TiO 2 and excimer-222/H 2 O 2 with DP (222 nm) of the RB5 dye, it is found that the degradation rate of RB5 is 52 times faster in excimer-222/H 2 O 2 case, whereas in the excimer-222/TiO 2 the degradation rate of RB5 decreased and it is only 2.9 times faster than DP.

Effect of suspension pH
The pH of the treated suspension is a critical factor that significantly impacts the degradation of contaminants 46 .
Three different pH values i.e. pH 2, pH 6, and pH 10 (solution) have been selected to see their impact on the degradation at a fixed TiO 2 dosages of 1 g/L.The pH values are changed from alkaline (pH 10) to acidic (pH 2) by adding H 2 SO 4 in the suspension.The impact of pH on the degradation of RB5 dye under acidic, ambient, and alkaline environments can be seen in Fig. 6f.The figure demonstrates that degradation occurs 8.27 times quicker in an alkaline medium compared to an acidic medium.The degradation rate constants of RB5 at pH values of 10 and 6 are 6.88 10 -4 cm 2 /mJ and 1.41 10 -3 cm 2 /mJ, respectively.These findings indicate that the degradation process occurs at a rate that is twice as fast in an alkaline medium compared to the neutral pH conditions.The noticed behaviour can be attributed to the enhanced production of •OH due to the increased concentration of hydroxide ions in the alkaline medium.A comparable outcome had been observed earlier in the degradation of the RB5 dye and acid blue 40 dye 46 .

Effect of dye concentration with all other parameters
The degradation time profiles with the different concentrations of pollutants have also been examined.The degradation process is evaluated by changing the dye concentration from 10 to 90 mg/L at optimized operating parameters and a pH of 10.  www.nature.com/scientificreports/and 90 mg/L initial RB5 dye concentrations.Degradation via DP using LPUV at 254 nm takes a longer time as compared to DP of RB5 dye at 222 nm.This is due to the fact that the molar absorption coefficient and quantum yield of RB5 is higher at 222 nm than at 254 nm.50 mg/L RB5 dye decomposed fully with a fluence of 2400 mJ/ cm 2 , whereas in the case of 90 mg/L it is degraded only ∼ 65% at the same UV fluence.The degradation of RB5 dye (50 mg/L) using a DBD Kr/Cl 2 excimer source with 1 g/L TiO 2 requires a duration of 60 min, whereas in the case of LPUV/TiO 2 it takes 70 min.The addition of TiO 2 with excimer 222 and LPUV does not show much difference in the degradation because of almost similar absorption coefficient of TiO 2 at 222 nm and 254 nm.In contrast, when H 2 O 2 was introduced in the dye suspension and exposed to DBD Kr/Cl 2 excimer source, it showed exceptional results, and complete degradation took only 9 min, whereas in the case of LPUV/H 2 O 2 it takes 35 min.The observed behaviour might be attributed to the increased generation of •OH in excimer-222/H 2 O 2 , ascribed to the higher molar absorption coefficients of H 2 O 2 at 222 nm.Table 1 shows a comparative analysis of RB5 degradation using various UV/AOP-based reactors operating at different power ratings.It reveals that the dye degradation rate is higher in excimer-222/ H 2 O 2 compared to the other UV based reactors reported earlier in the literature.

EEO and COD Analysis
When evaluating the potentiality of UV/AOP-based wastewater treatment techniques for commercialization, electrical energy per order (EEO) is an essential indicator.In this study, the EEO of the DBD Kr/Cl 2 excimer source is computed for different treatment methods using Eq. 5.For the complete degradation of RB5, the EEO in the case of excimer-222/TiO 2 is found to be 85.6 kWh/m 3 for 50 mg/L dye concentration and 1 g/L TiO 2 , whereas the EEO in the case of excimer-222/H 2 O 2 is found to be 12.83 kWh/m 3 for 50 mg/L dye concentration and 10 ppm H 2 O 2 .Figure 9a shows the EEO of the developed reactor for different treatment methods used in this work.Elbadawy et al. 47 used TiO 2 nano photocatalysts with conventional UV-C light (4.0 mW/cm 2 ) for the degradation of acid red 37 dye and reported a maximum EEO of 714 kWh/m 3 .The present excimer-UV/H 2 O 2 system has shown a reasonably lower EEO than many other reported UV/AOP reactors (see Table 1), indicating its potential for industrial exploitation.
Chemical oxygen demand analysis is employed to determine the mineralization of treated wastewater by comparing the untreated and treated wastewater solutions.The COD concentration of untreated dye solution (50 mg/L) and treated with LPUV DP, excimer-222 DP, excimer-222/TiO 2 , and excimer-222/H 2 O 2 at optimized parameters are shown in Fig. 9b.COD values are measured only after the 60 min of treatment time, to set a better baseline for comparison.It was found that the initial concentration of COD was 248 mg/L, and no significant change in COD concentration was observed when the dye solution was treated with LPUV DP and excimer-222 DP.When treated with excimer-222/TiO 2 , the COD concentration declined to 160.32 mg/L, which corresponds to 35.35% mineralization of RB5 dye.The process of excimer-222/H 2 O 2 shows some promising results as the rate of mineralization increases up to 91.62% in 60 min of treatment time (as shown in Figure S4).In the case of the excimer-222/H 2 O 2 process, COD is decreased by 47.7% after a treatment duration of 9 min, when the rate of decolourization of RB5 dye was determined to be 99.9%.Achieving a COD decrease of over 90% will require 60 min, given a dye concentration of 50 mg/L and 10 ppm H 2 O 2 .It has been shown that an increase in treatment time or UV dose is required to achieve more mineralization.Moreover, the RB5 dye suspension treated with an excimer-222/H 2 O 2 exhibited a notable reduction in COD values, indicating its reactive azo dye degradation ability.The findings show considerable potential for the rapid mineralization of textile wastewater, thereby presenting a viable approach for treating such effluent.

Degradation Pathway of RB5 dye
The present study investigates the degradation mechanism of RB5 dye by utilising HR-MS and FTIR techniques using untreated and excimer-222/H 2 O 2 treated wastewater samples.Fragmented by-products are identified through the utilisation of fragmentation data and m/z values obtained through mass spectrometry analysis.A potential mechanism for the degradation of RB5 dye has been proposed and illustrated in Fig. 10, based on the intermediates of its degradation and previous publications 51,52 .The degradation process of RB5 dye involves several sequential processes, including the dissociation of the azo (-N=N-) bond, as observed in FTIR analysis at 1458 cm −1 in both untreated and treated samples.Additionally, the functional group undergoes rearrangement, the ring is opened, and the dye undergoes mineralization.Figure S5  The C-O bond in RB5 dye is initially broken due to the direct interaction between the •OH and organic pollutants, resulting in the formation of an intermediate, depicted as A in Fig. 10.Moreover, the dissociation of the C-SO 3 link leads to the creation of product D through the fragmentation of product B. Furthermore, fragmentation of different intermediate products results in the formation of oxalic acid through the ring opening mechanism.The benzene rings underwent an •OH attack, resulting in their fragmentation.Consequently, the RB5 dye underwent degradation into intermediate molecules with lower resilience, ultimately leading to the formation of CO 2 and H 2 O by mineralization.The degradation of dyes can be inferred to involve the cleavage of azo linkages, leading to the generation of aromatic amines, subsequently resulting in their full disintegration 51,53 .The data analysis revealed that the azo (N=N) bond present in the RB5 dye has undergone degradation, forming many intermediate products.

Toxicity analysis and reusability of treated wastewater for agriculture purposes
The germination ability of untreated and treated artificial wastewater is assessed by directly sowing Raphanus sativus seeds into the soil.The germination of the Raphanus sativus seeds was carried out using different water samples, including deionized water, artificial wastewater, LPUV DP, excimer-222 DP, excimer-222/TiO 2 (1 g/L), and excimer-222/H 2 O 2 (10 ppm) treated dye wastewater.The germination process can occur for three days in a controlled environment.The analysis and comparison of seed germination ability of treated effluent with various methods is conducted by measuring the length of shoots and roots.The germination percentage, root length, and shoot length after a three-day incubation period are summarised in Fig. 11a and b.
The suspension resulting from excimer-222/TiO 2 and excimer-222/H 2 O 2 processes shows positive effects on Raphanus sativus seed germination and growth when compared to untreated artificial wastewater, LPUV DP and excimer 222 DP.The enhanced rate of germination was recorded in the excimer-222/H 2 O 2 process because of the complete mineralization of RB5 dye.The seed germination rate for excimer-222/ H 2 O 2 treated wastewater is 96%, but for excimer-222/TiO 2 , it is approximately 88% despite the same treatment duration.The excimer-222/ TiO 2 treated wastewater may contain a higher concentration of non-degraded dye and catalyst elements than the excimer-222/H 2 O 2 treated wastewater.The root and shoot length in dye wastewater is significantly reduced due to nutritional deficiency caused by the high concentration of organic pollutants.www.nature.com/scientificreports/textile effluent, this study evaluated the effects of 222 nm exposure on dye degradation in the presence of varying dye concentrations, catalyst loading, and pH levels.
In the conventional AOPs, the use of 254 nm sources poses a challenging problem because of mercury content, which is even more detrimental to the environment than the treated pollutants.Another concern related to these lamps is their routine replacement and often filament failure 54 .The excimer sources are considered as the best alternative for the conventional UV lamps and proven to be an efficient source for water treatment applications 44,45 .In this study, we have designed and developed a mercury free 222 nm excilamp by using the concept of plasma generation in dielectric barrier discharge (DBD) configurations.It has been observed that the developed DBD Kr/Cl 2 excilamp has an instant start-up with full radiation output ( ∼ 2.0 mW/cm 2 ), in con- trast to the mercury-based UV-C lamp, which has a start-up time of about 2.5 min 55 .The thermal conductivity of the treated suspension due to various reactive species can also be estimated by using numerical modelling techniques 56,57 .The Kr/Cl 2 excilamp is positioned above the water level, which has produced two important benefits.One benefit is that it prevents the fouling of the excilamp.Another benefit, as described by Rao et al. 58 , is the increased formation of •OH.This is due to the accelerated photolysis of H 2 O 2 occurring at the air-water interface rather than in the volume or bulk phase.
The intensity and wavelength of the UV source significantly influence the photocatalytic activity and dye degradation efficiency 59 .This study reveals that for the higher dye concentration, longer treatment time or higher fluence rate is required (see Fig. 8).As the dye concentration increases, the number of excimer UV-C (222 nm) photons that can reach the surface of the photocatalyst decreases, leading to a decrease in the formation of •OH 59 .Some studies also suggest that degradation/mineralization of suspension occurs by direct transfer of electrons and holes from the TiO 2 surface to the dye molecules 46,60 as, From Eqs. ( 9) and (10), it is clear that there is a negative correlation between dye concentration and photocatalyst activation.Specifically, as the dye concentration increases, the availability of electrons for direct reduction and holes for direct oxidation of the dye molecule decreases.
The increase in catalyst loading in the dye suspension increases the rate of degradation because the intensity of 222 nm radiation absorbed by TiO 2 increases with increasing TiO 2 dosage in the suspension.Interestingly, when the TiO 2 dose exceeded 1 to 1.25 g/L, the degradation rate constant declined.This is due to the fact that an increased concentration of TiO 2 in the suspension increases the reflectance of far UV-C light, which is mostly responsible for the decline in the RB5 dye degradation efficiency, as reported elsewhere 61 .Despite of having a reasonable degradation efficiency, TiO 2 photocatalysis has two main disadvantages: a high rate of recombination of electron-hole pair and the need for additional process of photocatalysts removal from the treated wastewater that is used in nano-powder form, which increases the overall cost of the treatment.The utilization of catalyst in wastewater suspensions is constrained by issues such as particle aggregation, slurry formation, and the expenses associated with catalyst separation.Thus, to address these constraints, magnetically retrievable catalysts and catalytic immobilization in the form of films or coatings on stationary supports have been employed 62 .The use of this method on a large scale can be limited by the need to replace the immobilized catalysts after certain cycles, as well as by mass transfer limits on the immobilized catalysts and the low quantum yield for •OH radical generation 63 .
This study has further utilized H 2 O 2 as a radical oxidant in the wastewater suspension.In the case of excimer-222/TiO 2 , time required for the complete degradation of RB5 was 60 min, but in the case of excimer-222/ H 2 O 2 , the time required for the degradation of RB5 is only 10 min.These exceptional results obtained in the case of H 2 O 2 addition are due to the powerful oxidizing nature of H 2 O 2 , reacting rapidly with highly energetic photons of far UV-C light to produce •OH through the following reaction 46 , In addition, the background water matrix plays significant role in case of 222 nm radiation.Nitrates presents in wastewater absorbs light extensively at 222 nm and produces •OH at the quantum yields of 9-21% by the process as mentioned in Eq. ( 10) 64 Excimer-222/H 2 O 2 process dominates in wastewater degradation and mineralization because of the following interactions: The addition of TiO 2 and H 2 O 2 together reduces the degradation efficiency because H 2 O 2 scavenges the photogenerated oxidizing species, i.e., •OH and h + vb (see Eqs. 14-16) 65 would otherwise be available for the oxidative destruction of the dye molecules.Excimer-222/H 2 O 2 is regarded as a viable approach for wastewater treatment due to its tendency to generate •OH and its sludge-free operation, characterised by accelerated kinetics 66 .This study reveals that the degradation of dye is pH sensitive and the highest rate of degradation has been found in the alkaline medium (pH 10) than the acidic and neutral medium.The higher production of hydroxyl radical in the alkaline medium could have increased the reaction rate.In general, the pH of the textile effluents is found alkaline ( ∼ 10 ± 1) 67 and the obtained results hold a good opportunity to degrade the textile effluents in a more effective way without the need of the neutralization.
Electrical energy per order (EEO) and COD tests are conducted for several process combinations (LPUV DP, Excimer 222 DP, LPUV/TiO 2 , Excimer-222/ TiO 2 , LPUV/H 2 O 2 , Excimer-222/ H 2 O 2 ), in addition to dye degradation.The optimized values, at which the highest percentage degradation (99.9%,) and COD removal (91.62%) was attained with low EEO of 12.84 kWh/m 3 is: H 2 O 2 concentration-10 ppm, UV 222 intensity-1.054mW/ cm 2 , and pH-10.The transformation of organic compounds into inorganic compounds directly impacts the reduction of COD and is directly related to the breakdown of organic matter 7 .The decrease in COD value is basically due to the excessive presence of •OH, and due to its higher oxidation potential (2.8 V), a higher degree of mineralization has been achieved 68 .
Understanding how various reactive species and 222 nm radiation cause degradation is essential for process standardization.The •OH is the major dye degrading agent and there are many ways to generate •OH, e.g., by the decomposition of H 2 O 2 under excimer 222 exposure, and by the reaction of ozone and/or UV 222 light with the water molecules.The proposed reaction mechanism of RB5 degradation with 222 excilamp is shown in Fig. 12 and briefly described as follows: 1. Formation of •OH by the action of UV 222 light with H 2 O 2 and water molecules (Eq.11) (most probable); 2. Formation of •OH by the photolysis of nitrates (already presents in the wastewater matrix) under 222 nm (Eq.12 and 13).3. Degradation of dye molecules with the direct photolysis with 222 nm photon due to high quantum yield of RB5 at 222 nm. 4. Reaction of dissolved ozone with the water molecules (least probable); 5.At last, degradation and mineralization of dye molecules via the oxidation of •OH (Eq.17).
The findings indicate the potential use of DBD Kr/Cl 2 excilamp in the context of wastewater treatment and its possible application in agriculture.Additional research is required to investigate the mechanisms responsible for these outcomes to improve the treatment method for broader use.

Industrial relevance
The developed process holds good promises for its use in the industrial wastewater treatment.Several tangible applications demonstrating the significance of research undertaken include the followings: 1. Wastewater treatment the use of Kr/Cl 2 excilamp can be an efficient way to clean wastewater that contains dyes from different industries, such as the production of paper, leather, and textiles.By aiding in the degradation and breakdown of dyes, this method can improve the safety of water for discharge into the environment or later reuse. (

( 1 )Figure 1 .
Figure 1.(a): 2-D schematic and original view of the KrCl* (222 nm) excimer light source.(b) Experimental setup of developed DBD Kr/Cl 2 excilamp with all electrical and optical measuring equipment.The optical emission spectra and UV 222 intensity are recorded from the centre of the Kr/Cl 2 excilamp.
Germination percentage = Number of seeds germinated Number of seeds sown × 100

Figure 2 .
Figure 2. (a) Typical voltage and current waveforms of the DBD Kr/Cl 2 excilamp, (b) OES of Kr/Cl 2 excilamp, (c) The absolute UV intensity of the Kr/Cl 2 excilamp with respect to discharge power, and (d) The relationship between the temperature change of the treated suspension and the excimer 222 nm fluence rate.

Figure 4 .
Figure 4. FE-SEM image of the synthesized (a) TiO 2 nanoparticles, (b) histogram distribution of TiO 2 nanoparticles for calculating average particle size.High-resolution XPS spectrum of (c) Ti 2p and (d) O1s in synthesized TiO 2 .

Figure 8 Figure 7 .
Figure 7. Photocatalytic mechanism action of TiO 2 in organic pollutants treatment under the irradiation of Kr/ Cl 2 excilamp.

Figure 9 .Figure 10 .
Figure 9. (a) Electrical energy per order corresponding to the various treatment methods for complete degradation of RB5 dye (50 mg/L) and (b) COD removal as a function of different treatment methods (C 0 = 50 mg/L, 1 g/L TiO 2 , and 10 ppm H 2 O 2 and pH 10).

Figure 11 .
Figure 11.(a) Box plot of the Kruskal-Wallis test for values of germination percentage of the Raphanus sativus seeds obtained from different treatment methods, (b) shoot and root length of Raphanus seeds after 3 d.

A
DBD-based far UV-C (222 nm) Kr/Cl 2 excilamp has been designed and developed.The advantage of the developed source is demonstrated for the degradation of complex organic molecules like RB5 dye, in DP and in AOP processes using TiO 2 /H 2 O 2 .The exceptional performance of the developed excilamp in the degradation of RB5 dye is achieved due to the higher molar absorption coefficient of RB5 dye at 222 nm than 254 nm.The quantum yield of RB5 dye at excimer-222/H 2 O 2 is found 31 times higher as compared to excimer-222/TiO 2, demonstrating the effectiveness of excimer-222/H 2 O 2 over other processes.It is found that the degradation rate of RB5 is ∼ 27 times faster in excimer-222/H 2 O 2 process, whereas in excimer-222/TiO 2 it is only 4 times faster as compared to excimer-222 DP.The order of degradation/mineralization is found to be excimer-222/H 2 O 2 > LPUV/ H 2 O 2 > excimer-222/TiO 2 > LPUV/TiO 2 > excimer-222 DP > LPUV DP.The EEO of the excimer-222/H 2 O 2 processis ∼ 500% higher than the excimer-222/TiO 2 process.The process that has been developed is environmentally friendly due to the absence of mercury usage, and it effectively removes the need for catalyst recovery from the treated effluent following degradation.The water resulting from these different treatments has been used for the germination of Raphanus sativus seed and it has shown a positive impact on the germination percentage, root length, and shoot length than untreated wastewater, demonstrating its non-toxic nature.The increased operation flexibility with lower EEO makes this technology a potential alternative to conventional LPUV-based AOPs for commercial exploitation.To make sure the reactor works for industrial purposes, more investigation and thorough pilot scale testing of the design are required.More research on the use of far UV-C excimer sources for sustainable agricultural techniques is also necessary.

Table 1 .
displays the FTIR analysis results of untreated and treated (excimer 222/H 2 O 2 ) wastewater samples.Comparative analysis of various UV-based reactors for the degradation of RB5 dye.
) H 2 O 2 + 2h + vb → O 2 + 2H + (17) RB5 + •OH → CO 2 + H 2 O + Byproducts Figure 12.Proposed reaction mechanism of RB5 degradation with 222 nm excimer source.2. Environmental remediation the use of Kr/Cl 2 excilamp can reduce contamination in the environment caused by dye spills or improper disposal practices.An important issue that requires attention is mercury discharge into the open environment, which can be prevented with the use of mercury-free Kr/Cl 2 excilamp.3. Textile industry integrating Kr/Cl 2 excilamp into textile manufacturing processes can serve as a means to purify wastewater before its release.This can reduce the ecological effect of textile manufacture and can ensure compliance with environmental regulations.